The present invention relates to a perpendicular magnetic recording medium and more particularly to a double-layered perpendicular recording medium comprising a substrate, a soft magnetic layer having high permeability formed on the substrate, and a pair of recording layers formed on the soft magnetic layer which are different in coercive force. This recording medium is suitable for use in a perpendicular magnetizing recording and reproduction system.
Conventionally varieties of perpendicular magnetic recording media have been proposed. For example, a perpendicular magnetic recording medium comprising a substrate, a soft magnetic layer formed on the substrate, and a single recording layer formed on the soft magnetic layer is disclosed in U.S. Pat. No. 4,210,946; a perpendicular magnetic recording medium comprising a substrate, and a recording layer formed thereon consisting of two layers having different coercive forces in U.S. Pat. No. 4,624,894; and a perpendicular recording media comprising a substrate and a recording layer whose coercive force continuously changes in the direction of the depth thereof in Japanese Laid-Open Patent Application No. 57-179942.
One of the key features of the perpendicular magnetic recording media is that they can attain much higher recording density than the conventional longitudinal magnetic recording media. Because of this feature of the perpendicular magnetic recording media, when information is recorded in the perpendicular magnetic recording media by a magnetic head, it is required that recording be carried out with higher recording frequency than in the case of the longitudinal magnetic recording media. Furthermore when recording is performed in such magnetic recording media by use of a magnetic head, the magnetic head is magnetized by a coil wound around the head with electric current caused to flow through the coil, and a magnetic field is applied to the recording media by the magnetized head. Therefore, the higher the recording frequency, the higher the impedance of the magnetic head, and accordingly the more limited the electric current that can be caused to flow through the magnetic head. For this reason, it is desired that high reproduction output will be obtained by small recording magnetomotive force.
The above-mentioned conventional perpendicular recording media, however, have the following shortcomings. More specifically, in the case of the perpendicular magnetic recording medium comprising a substrate, a soft magnetic layer formed on the substrate, and a single recording layer formed on the soft magnetic layer as disclosed in U.S. Pat. No. 4,210,946, the relationship between the coercive force of the recording layer and the reproduction output of the recording media is shown in FIG. 1a. As shown in FIG. 1a, as the coercive force of the recording layer is increased, the reproduction output of the recording medium also increases up to a certain value of coercive force. However, when the coercive force exceeds the certain value, the reproduction output decreases. This phenomenon may be explained as follows: By increasing the coercive force of the recording layer to a certain value, the reproduction output of the recording medium can be increased. This is because the effect of the demagnetizing field which works in the perpendicular direction of the recording medium can be reduced. However, when the coercive force of the recording medium is increased and exceeds a certain value, the coercive force of the medium itself is so high that the recording medium cannot be magnetized sufficiently for producing high reproduction output.
The magnetomotive force necessary for recording information in the recording medium increases in proportion to the increase in the coercive force of the recording medium as shown in FIG. 1b.
For example, when an output required for reproduction of recorded information, which will be simply referred to as the required reproduction output, is set as indicated by the broken line in FIG. 1a, and an allowable recording magnetomotive force is set as indicated by the broken line in FIG. 1b, the coercive force Hc required for obtaining the necessary reproduction output has to be in the range of Hc2 and Hc3, that is, Hc2.ltoreq.Hc.ltoreq.Hc3. However, as can be seen from FIG. 1b, the magnetomotive force which is more than the allowable magnetomotive force is necessary in this range. On the other hand, when the coercive force is less than the coercive force Hc1 at which the allowable magnetomotive force is satisfied, the necessary reproduction output cannot be obtained as can be seen from FIG. 1a.
In the case of the perpendicular magnetic recording medium comprising a substrate, and a recording layer which is directly formed thereon and consists of two layers having different coercive forces as disclosed in U.S. Pat. No. 4,624,894, as shown in FIG. 2a, the reproduction output decreases monotonously as the coercive force ratio Hcu/Hcl increases, in which Hcu is the coercive force of the layer on the free surface side of the recording layer, which is constant, and Hcl is the coercive force of the layer on the side of the substrate. This phenomenon may be explained as follows. In this recording medium, a substantially uniform demagnetizing field is applied to the recording medium in the perpendicular direction thereof. Therefore, the reproduction output may be expressed as the sum of the reproduction output of each layer, so that as Hcl decreases, the contribution of the lower layer to the total reproduction output of the recording medium also decreases. Furthermore, since the recording magnetomotive force of the recording medium is determined by the sum of the magnetomotive force of each layer, when Hcu/Hcl becomes large, the required magnetomotive force decreases. Therefore, for instance, when a required reproduction output is set as indicated by the broken line in FIG. 2a, and an allowable recording magneto-motive force is set as indicated by the broken line in FIG. 2b, there exits no area where both the reproduction output and the allowable magnetomotive force are satisfied.
In both of the above-mentioned conventional perpendicular magnetic recording media, when the recording magnetomotive force decreases, the reproduction output also decreases.